Abstract:

A liquid tank, a viewing device for under-liquid observation, and an
optical film are disclosed, each excellent in visibility and durability
and being easy to clean. In at least one embodiment of the present
invention is directed to a liquid tank having a transparent wall,
including, on an internal surface of the wall, a first moth-eye layer
having a moth-eye structure, and a protective layer covering the moth-eye
structure, in this order from the wall side, and preferably, further
including, on an external surface of the wall, a second moth-eye layer
having a moth-eye structure, and the second moth-eye layer being disposed
in a region facing the first moth-eye layer.

Claims:

1. A liquid tank having a transparent wall,comprising, on an internal
surface of the wall,a first moth-eye layer having a moth-eye structure,
anda protective layer covering the moth-eye structure, in this order from
the wall side.

2. The liquid tank according to claim 1,further comprising, on an external
surface of the wall,a second moth-eye layer having a moth-eye structure,
andthe second moth-eye layer being disposed in a region facing the first
moth-eye layer.

3. The liquid tank according to claim 1,wherein the protective layer
contains a fluorine-containing resin.

4. The liquid tank according to claim 1,wherein the protective layer has a
refractive index substantially equivalent to that of a liquid to be
stored in the tank.

5. The liquid tank according to claim 1,wherein the protective layer has a
refractive index substantially equivalent to that of water.

6. The liquid tank according to claim 1,wherein the protective layer has a
refractive index smaller than that of each of the transparent wall and
the first moth-eye layer.

7. The liquid tank according to claim 1,wherein the protective layer has a
refractive index of 1.28 to 1.38.

8. An optical film attached to an internal surface of a liquid tank having
a transparent wall,the film comprising a moth-eye layer having a moth-eye
structure and a protective layer covering the moth-eye structure.

9. The optical film according to claim 8,wherein the protective layer
contains a fluorine-containing resin.

10. The optical film according to claim 8,wherein the protective layer has
a refractive index substantially equivalent to that of a liquid to be
stored in the tank.

11. The optical film according to claim 8,wherein the protective layer has
a refractive index substantially equivalent to that of water.

12. The optical film according to claim 8,wherein the protective layer has
a refractive index smaller than that each of the transparent wall and the
moth-eye layer.

13. The optical film according to claim 8,wherein the protective layer has
a refractive index of 1.28 to 1.38.

14. A viewing device having a transparent window for under-liquid
observation,the device comprising, on an outer surface of the window, a
first moth-eye layer having a moth-eye structure, anda protective layer
covering the moth-eye structure, in this order from the window side.

15. The viewing device according to claim 14,further comprising, on an
inner surface of the window, a second moth-eye layer having a moth-eye
structure, andthe second moth-eye layer being disposed in a region facing
the first moth-eye layer.

16. The viewing device according to claim 14,wherein the protective layer
contains a fluorine-containing resin.

17. The viewing device according to claim 14,wherein the protective layer
has a refractive index substantially equivalent to that of a liquid to be
in contact with the device.

18. The viewing device according to claim 14,wherein the protective layer
has a refractive index substantially equivalent to that of water.

19. The viewing device according to claim 14,wherein the protective layer
has a refractive index smaller than that of each of the transparent
window and the first moth-eye layer.

20. The viewing device according to claim 14,wherein the protective layer
has a refractive index of 1.28 to 1.38.

21. An optical film attached to an outer surface of a viewing device
having a transparent window for under-liquid observation,the film
comprising a moth-eye layer having a moth-eye structure and a protective
layer covering the moth-eye structure.

22. The optical film according to claim 21,wherein the protective layer
contains a fluorine-containing resin.

23. The optical film according to claim 21,wherein the protective layer
has a refractive index substantially equivalent to that of a liquid to be
in contact with the device.

24. The optical film according to claim 21,wherein the protective layer
has a refractive index substantially equivalent to that of water.

25. The optical film according to claim 21,wherein the protective layer
has a refractive index smaller than that of each of the transparent
window and the moth-eye layer.

26. The optical film according to claim 21,wherein the protective layer
has a refractive index of 1.28 to 1.38.

Description:

TECHNICAL FIELD

[0001]The present invention relates to liquid tanks, viewing devices for
under-liquid observation, and optical films. More particularly, the
present invention relates to: a liquid tank such as an aquarium tank and
a tank with a liquid volume meter, suitably used as a container the
inside of which is viewed through a transparent wall; a viewing device
for under-liquid observation suitably used as a viewing device for
underwater observation; and an optical film for the liquid tank or the
viewing device.

BACKGROUND ART

[0002]The liquid tank includes an aquarium or observation container, and
particularly tanks for raising and breeding fishes and the like
thereinside are common. Such tanks are usually composed of a transparent
material. In an aquarium tank containing water 21 as shown in FIG. 9, for
example, ambient light is reflected on the external and internal (outer
and inner surfaces) of an observer-side transparent plate (the
transparent wall 11), and this makes it difficult for the observer to
clearly view the inside of the tank and also to photograph it clearly
because strobe light is reflected. With a view to clearly observe
aquarium or food fishes raised or bred in the tank from the outside,
modifications and improvements have been made not only for the tank
itself such as a viewing window and a transparent wall but for
maintenance tools installed on the tank, including an air pump and a
cleaning tool.

[0003]For example, as a way of more clearly viewing underwater objects, it
is commonly known that an anti-reflection thin film is coated on the
external surface of a glass or acrylic resin transparent wall
constituting a tank, or alternatively, an anti-reflection film is
attached to it, thereby preventing reflection of ambient light on the
external surface of the wall (hereinafter, also referred to as "external
surface-reflection").

[0004]With regard to a technology for preventing reflection on an
interface between a content of the tank and the internal surface (the
interior of the tank) of the wall (hereinafter, also referred to as
"internal surface-reflection") in addition to the external
surface-reflection, for example, Patent Document 1 discloses that at
least an observer-side one of the transparent walls constituting the tank
has an anti-reflection function on its both surface.

[0005]Examples of the liquid tank also include a tank equipped with a
window (viewing window) used as a liquid volume meter and through which
objects present in the liquid can be viewed. Also in such a tank,
reflection on the external and internal surfaces of the viewing window
occurs in the matter of the above-described tank, so that an observer has
a difficulty in accurately measuring the volume of the liquid.

[0006]In the tank inside which fishes are raised and bred, there is
required to minimize or completely prevent adhesion of algae to the wall
surface, thereby maintaining the wall surface clean and keeping the
fishes in captivity healthy. As a technology of achieving it, for
example, Patent Document 2 discloses a film for lining a water tank,
provided with a tacky layer having the transparency and water resistance
on one surface of a transparent resin film, further a release sheet or a
sheet of paper on the surface thereof and a peeling off means for peeling
at least one end of the release sheet or paper.

[0007]Similarly in the liquid tanks, the clear under-liquid observation is
difficult also in viewing devices for under-liquid observation, including
goggles and aquascopes. In goggles, for example, reflection on a lens
surface might occur.

[0008]As a technology of preventing such surface reflection, a "moth-eye
structure", a kind of nanostructure, is known in the field of optical
materials. The moth-eye structure includes a great number of
nanometer-sized conical protrusions formed on a transparent substrate
surface, for example. As disclosed in Patent Documents 3 to 6, the
moth-eye structure allows continuous change of the refractive index from
air to the transparent substrate, so that incident light does not
recognize the interface as an optical surface, and as a result,
reflection of the light can be significantly decreased.

[Patent Document 1]

[0009]Japanese Kokai Publication No. 2003-319733

[Patent Document 1]

[0009][0010]Japanese Kokai Publication No. Hei-09-322674

[Patent Document 1]

[0010][0011]Japanese Kokai Publication No. 2003-43203

[Patent Document 1]

[0011][0012]Japanese Kokai Publication No. 2005-156695

[Patent Document 1]

[0012][0013]WO 2006/059686

[Patent Document 1]

[0013][0014]Japanese Kokai Publication No. 2001-264520

DISCLOSURE OF INVENTION

[0015]When optical films having a surface with a nanostructure such as a
moth-eye structure are used under water or another liquid of the tank,
under-liquid life such as microorganism and algae tend to adhere to the
film surface. This might look like stains and deteriorate the appearance,
and further lead to a reduction in transmittance. For example, when a
moth-eye structure 16 is used, a stain 31 such as underwater life is
fixed to the features (the moth-eye structure 16) as illustrated in FIG.
10, and this stain is very difficult to remove, for example, even by
washing with a cloth and the like because a fiber 32 of the cloth is
generally much larger than the structure 16. Also in a viewing
window-including tank or a viewing device each including the optical
film, stains are easy to adhere to the film but it is difficult to remove
them therefrom. When the technology disclosed in Patent Document 2 is
employed as a measure against this stain, the release sheet or the sheet
of paper for preventing this stain is disposed on the moth-eye structure,
so that the anti-reflection function derived from the moth-eye structure
can not be exhibited. Further, ambient light is reflected on the
interface between media with different refractive indexes, i.e., water
and the release sheet or the paper, and the viewing is significantly
deteriorated as a result. Thus in the use of the nanostructure such as a
moth-eye structure for the purpose of preventing the surface reflection
on an aquarium or observation container, the film with the nanostructure
is required to prevent the surface reflection and further suppress the
stains that are generated due to the content of the container and to
allow easy removal of the stains. Also in the use of the nanostructure
such as a moth-eye structure for anti-surface reflection of the viewing
device, the film with the nanostructure is required to prevent the
surface reflection and further suppress the stains from adhering to a
transparent window of the viewing device and to allow easy removal of the
stains.

[0016]The technology of Patent Document 1 does not suffer from such a
problem relating to the stains peculiar to the nanostructure because
instead of the nanostructure, the multi-layer anti-reflection film is
used. However, this multi-layer structure leads to poor wavelength
dispersion characteristic of the anti-reflection film to cause coloring
of reflected light. Further, unlike the film with the moth-eye structure,
the multilayer anti-reflection film can not decrease the reflectance to
such a level that almost no surface reflection occurs. When disposed on
the internal surface of a transparent wall or the external surface of a
transparent window, for example, the multi-layer anti-reflection film is
readily damaged by hit of fishes to the wall or window. That is, the
multi-layer anti-reflection film usually has poor durability. As
mentioned above, in the use of the multi-layer anti-reflection film for
the purpose of anti-surface reflection, the film is required to provide
further improved visibility by suppressing the coloring and reducing the
reflectance and to show enhanced durability. The multi-layer
anti-reflection film is an expensive material and requires complicated
production steps, and therefore it might be lack in versatility.

[0017]The present invention is devised considering the aforementioned
situations. An object of the present invention is to provide a liquid
tank, a viewing device for under-liquid observation, and an optical film,
each excellent in visibility and durability and being easy to clean.

[0018]The present inventors made various investigations on a liquid tank
excellent in visibility and durability and easy to clean, and noted a
moth-eye structure. The inventors found that the following effects can be
achieved when not only the moth-eye structure but also a protective layer
covering it are disposed on the internal surface of a transparent wall
constituting the tank. First, the surface reflection, especially the
internal surface-reflection, can be markedly decreased while the coloring
is suppressed. Secondly, the durability on the internal surface of the
tank is improved. Finally, the internal surface of the tank can be
readily cleaned. Thus, the present inventors have found the solution of
the aforementioned problems and arrived at the present invention.

[0019]The present invention is a liquid tank having a transparent wall,
including, on an internal surface of the wall,

[0020]a first moth-eye layer having a moth-eye structure, and

[0021]a protective layer covering the moth-eye structure, in this order
from the wall side. This liquid tank is excellent in visibility and
durability and easy to clean. Further, the liquid tank can be produced at
a much lower cost than in use of other anti-reflection films such as a
multi-layer anti-reflection film.

[0022]The liquid tank of the present invention may or may not include
other components as long as it essentially includes the above-mentioned
components, and the configuration thereof is not especially limited.

[0023]The following will mention preferable embodiments of the liquid tank
of the present invention in detail. The following embodiments may be
appropriately employed in combination.

[0024]With a view to significantly decrease the external
surface-reflection and further improve the visibility, it is preferable
that the liquid tank further includes, on an external surface of the
wall, a second moth-eye layer having a moth-eye structure, and the second
moth-eye layer being disposed in a region facing the first moth-eye
layer.

[0025]For further improvement in durability, it is preferable that the
protective layer contains a fluorine-containing resin. The
fluorine-containing resin generally has a refractive index of about 1.3
to 1.4, so that the refractive index of the protective layer containing
such a resin can be readily made substantially equivalent to that of
water. That is, according to this, the liquid tank of the present
invention can be suitably used as a water tank.

[0026]It is preferable that the protective layer has a refractive index
substantially equivalent to that of a liquid to be stored in the tank.
According to this, the effects of the present invention can be more
surely exhibited.

[0027]It is preferable that the protective layer has a refractive index
substantially equivalent to that of water. According to this, the liquid
tank of the present invention can be suitably used as a water tank.

[0028]It is preferable that the protective layer has a refractive index
smaller than that of each of the transparent wall and the first moth-eye
layer. According to this, the transparent wall and the first moth-eye
layer can be composed of the respective appropriate materials, and the
refractive index of a liquid to be stored in the tank can be readily made
substantially equivalent to that of the protective layer.

[0029]It is preferable that the protective layer has a refractive index of
1.28 to 1.38. According to this, the liquid tank of the present invention
can be suitably used as a water tank.

[0030]The present invention is also an optical film attached to an
internal surface of a liquid tank having a transparent wall, the film
including a moth-eye layer having a moth-eye structure and a protective
layer covering the moth-eye structure (hereinafter, also referred to as a
"first optical film of the present invention"). The liquid tank of the
present invention can be easily and inexpensively provided by attaching
the first optical film of the present invention to the internal surface
of the transparent wall.

[0031]The first optical film may or may not include other components as
long as it essentially includes the above-mentioned components, and the
configuration thereof is not especially limited.

[0032]The following will mention preferable embodiments of the first
optical film of the present invention. The following embodiments may be
appropriately employed in combination.

[0033]For further improvement in durability, it is preferable that the
protective layer contains a fluorine-containing resin. The
fluorine-containing resin generally has a refractive index of about 1.3
to 1.4, so that the refractive index of the protective layer containing
such a resin can be easily made substantially equivalent to of water.
That is, according to this, the liquid tank including the first optical
film of the present invention attached thereto can be suitably used as a
water tank.

[0034]It is preferable that the protective layer has a refractive index
substantially equivalent to that of a liquid to be stored in the tank.
According to this, the effects of the present invention can be more
surely exhibited.

[0035]It is preferable that the protective layer has a refractive index
substantially equivalent to that of water. According to this, the liquid
tank including the first optical film of the present invention attached
thereto can be suitably used as a water tank.

[0036]It is preferable that the protective layer has a refractive index
smaller than that each of the transparent wall and the moth-eye layer.
According to this, the transparent wall and the moth-eye layer can be
composed of the respective appropriate materials, and the refractive
index of a liquid to be stored in the tank including the first optical
film of the present invention can be readily made substantially
equivalent to that of the protective layer.

[0037]It is preferable that the protective layer has a refractive index of
1.28 to 1.38. According to this, the liquid tank including the first
optical film of the present invention attached thereto can be suitably
used as a water tank.

[0038]The present invention is also a viewing device having a transparent
window for under-liquid observation,

[0039]the device including, on an outer surface of the window, a first
moth-eye layer having a moth-eye structure, and a protective layer
covering the moth-eye structure, in this order from the window side. This
viewing device is excellent in visibility and durability and easy to
clean. Further, the viewing device including such layers can be produced
at a much lower cost than in use of other anti-reflection films such as a
multi-layer anti-reflection film.

[0040]The viewing device of the present invention may or may not include
other components as long as it essentially includes the above-mentioned
components, and the configuration thereof is not especially limited.

[0041]The following will mention preferable embodiments of the viewing
device of the present invention. The following embodiments may be
appropriately employed in combination.

[0042]With a view to markedly decrease reflection of ambient light on the
internal surface of the transparent window to further improve the
visibility, it is preferable that the viewing device further includes, on
an inner surface of the window, a second moth-eye layer having a moth-eye
structure, and

[0043]the second moth-eye layer being disposed in a region facing the
first moth-eye layer.

[0044]For further improvement in durability, it is preferable that the
protective layer contains a fluorine-containing resin. The
fluorine-containing resin generally has a refractive index of about 1.3
to 1.4, so that the refractive index of the protective layer containing
such a resin can be easily made substantially equivalent to that of
water. That is, according to this, the viewing device of the present
invention can be suitably used as a device for underwater observation.

[0045]It is preferable that the protective layer has a refractive index
substantially equivalent to that of a liquid to be in contact with the
device. According to this, the effects of the present invention can be
more surely exhibited.

[0046]It is preferable that the protective layer has a refractive index
substantially equivalent to that of water. According to this, the viewing
device of the present invention can be suitably used as a device for
underwater observation.

[0047]It is preferable that the protective layer has a refractive index
smaller than that of each of the transparent window and the first
moth-eye layer. According to this, the transparent window and the first
moth-eye layer can be composed of the respective appropriate materials,
and the refractive index of a liquid to be in contact with the device can
be readily made substantially equivalent to that of the protective layer.

[0048]It is preferable that the protective layer has a refractive index of
1.28 to 1.38. According to this, the viewing device of the present
invention can be suitably used as a device for underwater observation.

[0049]The present invention is also an optical film attached to an outer
surface of a viewing device having a transparent window for under-liquid
observation,

[0050]the film including a moth-eye layer having a moth-eye structure and
a protective layer covering the moth-eye structure (hereinafter, also
referred to as a "second optical film of the present invention"). The
viewing device of the present invention can be readily and inexpensively
provided by attaching the second optical film of the present invention to
the external surface of the window.

[0051]The second optical film of the present invention may or may not
include other components as long as it includes the above-mentioned
components, and the configuration thereof is not especially limited.

[0052]The following will mention preferable embodiments of the second
optical film of the present invention. The following embodiments may be
appropriately employed in combination.

[0053]For further improvement in durability, it is preferable that the
protective layer contains a fluorine-containing resin. The
fluorine-containing resin generally has a refractive index of about 1.3
to 1.4, so that the refractive index of the protective layer containing
such a resin can be easily made substantially equivalent to that of
water. That is, according to this, the viewing device including the
second optical film of the present invention attached thereto can be
suitably used as a device for underwater observation.

[0054]It is preferable that the protective layer has a refractive index
substantially equivalent to that of a liquid to be in contact with the
device. According to this, the effects of the present invention can be
more surely exhibited.

[0055]It is preferable that the protective layer has a refractive index
substantially equivalent to that of water. According to this, the viewing
device including the second optical film of the present invention
attached thereto can be suitably used as a device for underwater
observation.

[0056]It is preferable that the protective layer has a refractive index
smaller than that of each of the transparent window and the moth-eye
layer. According to this, the transparent window and the moth-eye layer
can be composed of the respective optimum materials, and the refractive
index of a liquid to be in contact with the viewing device including the
second optical film of the present invention can be readily made
substantially equivalent to that of the protective layer.

[0057]It is preferable that the protective layer has a refractive index of
1.28 to 1.38. According to this, the viewing device including the second
optical film of the present invention attached thereto can be suitably
used as a device for underwater observation.

Effect of the Invention

[0058]The liquid tank and the first optical film of the present invention
can provide a liquid tank excellent in visibility and durability and easy
to clean. More specifically, the use of the liquid tank and the first
optical film of the present invention allows a marked decrease in
coloring and an improvement in durability of the internal surface of the
liquid tank, and further allows easily cleaning the internal surface of
the tank. Further, the refractive index of the protective layer can be
easily made substantially equivalent to that of a liquid to be stored in
the tank. As a result, the reflection on the surfaces, particularly on
the internal surface of the tank can be markedly decreased. In addition,
the protective layer and air are usually different in refractive index,
so that the liquid level in the tank can be readily determined by being
viewed from the outside through the transparent wall.

[0059]The viewing device and the second optical film of the present
invention can provide a viewing device for under-liquid observation
excellent invisibility and durability and easy to clean. More
specifically, the use of the viewing device and the second optical film
of the present invention allows a marked decrease in coloring and an
improvement in durability of the external surface of the viewing device,
and further allows easily cleaning the external surface of the device.
Further, the refractive index of the protective layer can be easily made
substantially equivalent to that of a liquid to be in contact with the
device. As a result, the reflection of ambient light on the surfaces,
particularly on the external surface, of the device can be markedly
decreased.

BRIEF DESCRIPTION OF DRAWINGS

[0060]FIG. 1 is a schematic view showing a liquid tank of Example 1. FIG.
1(a) is an overall cross-sectional view of the tank. FIG. 1(b) is an
enlarged view of a region (side surface) surrounded by the broken line of
FIG. 1(a).

[0061]FIG. 2 is a cross-sectional view schematically showing a moth-eye
layer of the liquid tank of Example 1.

[0062]FIG. 3 is a cross-sectional view schematically showing the moth-eye
layer and a protective layer in the liquid tank of Example 1.

[0063]FIG. 4 is an explanation view of refractive index change in the
liquid tank of Example 1. FIG. 4(a) is a side cross-sectional view of the
tank. FIG. 4(b) is a graph showing refractive index change on a side
surface of the tank.

[0064]FIG. 5 is a schematic view showing the liquid tank of Example 2.
FIG. 5(a) is an overall cross-sectional view of the tank. FIG. 5(b) is an
enlarged view of a region (side surface) surrounded by the broken line of
FIG. 5(a).

[0066]FIG. 7 is an explanation view of refractive index change in the
liquid tank of Example 3. FIG. 7(a) is a cross-sectional view of a liquid
volume meter portion of the tank. FIG. 7(b) is a graph showing refractive
index change in the liquid volume meter portion.

[0067]FIG. 8 is an explanation view of a moth-eye structure (the view for
explaining the principle of continuous refractive index change from the
surface and the marked reflection decrease on the interface). FIG. 8(a)
is a schematic view showing a cross-section of the moth-eye structure.
FIG. 8(b) shows refractive index change in the structure.

[0071]FIG. 12 is an explanation view of refractive index change in the
viewing device of Example 4. FIG. 12(a) is a cross-sectional view of the
device. FIG. 12(b) is a graph showing refractive index change in the
device.

[0072]FIG. 13 is a cross-sectional view schematically showing the device
of Example 4 in accordance with a modified example.

[0073]FIG. 14 is a cross-sectional view schematically showing the device
of Example 4 in accordance with another modified example.

[0075]FIG. 16 is an explanation view of refractive index change in the
device of Example 5. FIG. 16(a) is a cross-sectional view of the device.
FIG. 16(b) is a graph showing refractive index change in the device.

BEST MODES FOR CARRYING OUT THE INVENTION

[0076]The present invention will be explained in more detail below on the
basis of Embodiments, with reference to accompanying drawings. The
present invention, however, is not limited to these Embodiments.

[0077]In the present description, when one member has a refractive index
substantially equivalent to that of another member, the refractive
indices thereof is not necessarily completely the same and may be
substantially the same to such a degree that the effects of the present
invention are exhibited.

[0078]In the present description, the refractive index is expressed as a
value measured at a wavelength of 589 nm (D ray) at a room temperature
(25° C.)

Embodiment 1

[0079]The liquid tank of Embodiment 1 has a transparent wall, and on the
internal surface of the wall, a first moth-eye layer having a moth-eye
structure and a protective layer covering the moth-eye structure are
disposed in this order from the wall side.

[0080]Inside the tank of the present Embodiment, a material containing at
least a liquid is stored. An observer views the content of the tank, such
as under-liquid objects (e.g., living creatures), the liquid level, the
liquid itself from the outside through the transparent wall. Thus, the
tank of the present Embodiment is suitably used as an aquarium or
observation tank, including a water tank and a tank with a viewing
window.

[0081]According to the tank of the present Embodiment, the first moth-eye
layer having a moth-eye structure allows markedly reducing light
reflection on the interface between the first moth-eye layer and the
protective layer (e.g., a reflectance of 0.1% or lower). The transparent
wall and the liquid (hereinafter, also referred to as a "content liquid")
to be stored in the tank are usually different in refractive index. The
material for the first moth-eye layer can be appropriately selected from
those having a refractive index substantially equivalent to that of the
transparent wall. The material for the protective layer can be
appropriately selected from those having a refractive index substantially
equivalent to that of the content liquid. Specifically, the first
moth-eye layer ensures the refractive index compatibility with the
transparent wall, and the protective layer ensures that with the content
liquid. As a result, the refractive index can be continuously changed
from the content liquid to the transparent wall, so that the
internal-surface reflection in a region below the liquid level can be
markedly decreased (e.g., a reflectance of 0.1% or lower)

[0082]Patent Document 6 discloses a reflection preventing film that
includes a transparent layer composed of hardened matter of an ionizing
radiation-setting resin composition. On one surface of the transparent
layer, a structure provided with a projecting and recessing part on which
countless fine projections and recessions are formed with a pitch shorter
than the wavelength of light. Further, a layer (coating layer 4) composed
of a resin composition and having a refractive index lower than that of
the transparent layer is laminated on the fine projections and
recessions. It is also disclosed that the material with a refractive
index of 1.3 to 1.4 is used for the surface layer 4. However, reflection
of about 2% occurs on the interface between air (refractive index of 1)
and this surface layer 4 (refractive index of 1.35), for example, when
the layer 4 has a refractive index of 1.35. Thus, this anti-reflection
film is poor in the effect of reducing unnecessary reflection.

[0083]The anti-reflection mechanism derived from the moth-eye structure
excludes λ/4 conditions unlike in the use of the multi-layer
anti-reflection film, and therefore coloring can be almost completely
prevented.

[0084]The liquid tank includes the protective layer covering the moth-eye
structure in addition to the first moth-eye layer, and materials
excellent in durability can be appropriately determined as a material for
the protective layer. Accordingly, the durability of the internal surface
of the tank can be improved.

[0085]Since the moth-eye structure is covered by the protective layer,
adherence of stains to the moth-eye structure, from which the stains are
difficult to remove, can be prevented. The shape of a portion to be in
contact with the content liquid (surface shape) of the protective layer
is not especially limited, and therefore the surface can be formed to be
more flat than the moth-eye structure. Even when adhered to the
protective layer, the stains can be readily removed therefrom.

[0086]The refractive index of the protective layer, which is substantially
equal to that of the content liquid, is usually different from that of
air. In the region below the content liquid level, the reflective indices
of the respective members are made substantially equal to each other. In
the region above the level (generally the region exposed to air), the
refractive indices of the respective members are not matched and as a
result of this, the internal surface-reflection occurs. Based on this
difference in occurrence of the internal-surface reflection between the
regions below and above the content liquid, the position (level) of the
content liquid can be readily determined by being viewed from the outside
through the transparent wall.

[0087]The moth-eye structure of the present invention can be formed, for
example, by coating a UV-curable resin on a base film or a mold surface,
pressing the film against the mold, and curing the resin coating by UV
radiation through the base film. This allows producing the liquid tank
dramatically inexpensively in comparison to use of other anti-reflection
films such as a multi-layer film (multi-layer anti-reflection film)
requiring expensive processes involving two or more coating processes,
deposition processes, and the like.

[0088]It is preferable that the liquid tank includes, on the external
surface of the transparent wall, a second moth-eye layer having a
moth-eye structure, and the second moth-eye layer is disposed in a region
facing the first moth-eye layer. This second moth-eye layer allows
markedly reducing the external surface-reflection (e.g., a reflectance of
0.1% or lower). As a result of this, the reflection on the region where
the first and second moth-eye layers overlap with each other can be
further decreased. The first and second moth-eye layers are disposed so
as to overlap with each other in the viewing direction. Between the first
and second moth-eye layers, the shape, size, and location thereof may be
the same or different but preferably the same.

[0089]The location of the first and second moth-eye layers and the
protective layer is not especially limited as long as they can be viewed
from an observer. Examples thereof include a side surface, an upper
surface, and a bottom surface of the tank. The first and second moth-eye
layers and the protective layer may be formed over the entire or part of
the transparent wall, but preferably formed on at least part through
which the observer views the content of the tank. Specifically, when the
liquid tank of the present Embodiment is used in an application where the
viewing direction is predetermined, the first and second moth-eye layers
and the protective layer may be disposed only on the transparent wall
facing the observer, not on all the walls.

[0090]The moth-eye structure is not especially limited as long as it
includes a large number of nanometer-sized protrusions. Specifically, the
moth-eye structure includes, but is not especially limited to, a large
number of protrusions each having a height (depth) of 1 nm or larger and
smaller than 1 μm (=1000 nm). More specifically, the moth-eye
structure preferably includes protrusions whose pitch (a distance between
tops of adjacent ones of the protrusions) and height are each smaller
than a visible light wavelength (380 nm to 780 nm), and more preferably
includes substantially conical protrusions each of which has a pitch of
50 nm to 300 nm and a height of 50 nm to 300 nm. The thickness of each of
the first and second moth-eye layers is generally, but not limited to,
about 1 μm to 30 μm.

[0091]This surface structure is typically called "moth-eye structure" and
a film with this structure is known as an ultra-low reflection film
allowing a reflectance of 0.1% or lower for visible light, for example.
FIG. 8 is an explanation view of a moth-eye structure (the view for
explaining the principle of continuous refractive index change from the
surface and the marked reflection decrease on the interface). FIG. 8(a)
is a schematic view showing a cross-section of the moth-eye structure.
FIG. 8(b) shows refractive index change in the moth-eye structure. FIG. 8
shows the case where the moth-eye structure is exposed to air. The
moth-eye structure-including film typically has protrusions 19 smaller
than a visible light wavelength (380 nm to 780 nm) as shown in FIG. 8(a),
and as shown in FIG. 8(b), the interface refractive index can be regarded
to gradually and continuously increase from the refractive index of the
medium (air) on the film surface (refractive index of air is 1.0) to a
value equivalent to the refractive index of a material constituting the
film (for example, a refractive index of a resin is ordinarily about
1.5). As a result, no refractive index interface substantially exists,
and the reflectance on the interface is significantly decreased.

[0092]The moth-eye structure can be formed by nanoimprint, including UV
nanoimprint and thermal imprinting. Particularly preferably, an aluminum
substrate with nanometer-sized cavities formed on its surface by
anodization is used as a mold. More specifically, the mold with
nanometer-sized cavities can be prepared by cut-polishing an outer
circumference surface of a conical or cylindrical aluminum tube formed by
extruding, and alternately repeating, for example, three times,
anodization and etching for the cut-polished aluminum surface (outer
circumference surface). According to this method involving anodizing,
nanometer-sized recesses can be formed in a random placement and in a
uniform distribution, and seamless nanostructures needed for continuous
production can be formed on a columnar or cylindrical mold roller
surface. Accordingly, the seamless nanostructures can be continuously
transferred to a workpiece (for example, a UV-curable or thermocurable
resin film).

[0093]The materials for the first and second moth-eye layers are not
especially limited. It is preferable that the refractive index of each of
the first and second moth-eye layers is substantially equivalent to that
of the transparent wall. More specifically, the difference in refractive
index between the transparent wall and the first and second moth-eye
layers is preferably 0.05 (more preferably 0.03) or smaller, and in this
case, the reflection on the both interfaces can be hardly recognized.

[0094]The other optical characteristics of the first and second moth-eye
layers are not especially limited and may be the same as those of common
moth-eye films. The first and second moth-eye layers are preferably
transparent to such a degree that an observer can view the content of the
tank. More specifically, it is preferable that: the light transmittance
is 90% (more preferably 95%) or higher; the haze is preferably 10% (more
preferably 1%) or lower; the refractive index is 1.45 to 1.55 (more
preferably 1.47 to 1.53).

[0095]In view of readily preparing the first and second moth-eye layers by
the above-mentioned nanoimprint, resins that can be cured by energy
beams, e.g., electromagnetic waves such as UV light and visible light or
thermocurable resins are preferably used as the material for the first
and second moth-eye layers.

[0096]Each of the first and second moth-eye layers may be (1) formed on
the transparent wall with a base film therebetween or may be (2) directly
formed on the transparent wall.

[0097]According to the embodiment (1), an optical film including the
moth-eye structure-containing layer and the protective layer covering the
moth-eye structure; and an optical film having a moth-eye
structure-including layer can be preferably used. Specifically,
mass-produced films formed by nanoimprint can be used, which allows an
improvement in productivity of the liquid tank of the present Embodiment.

[0098]Another Embodiment of the present invention provides such an optical
film. The present Embodiment provides an optical film (film for the tank
internal surface) attached to the internal surface of the liquid tank of
Embodiment 1, i.e., the liquid tank having a transparent wall, the
optical film including a moth-eye layer having a moth-eye structure (the
above-mentioned first moth-eye layer) and a protective layer covering the
moth-eye structure (the moth-eye structure of the above-mentioned first
moth-eye layer).

[0099]The base film is a base for supporting a material (for example, a
UV-curable or thermocurable resin) for the (first and/or second) moth-eye
layer during the production steps. It is preferable that the base film
has transparency, and resistances to water and solvent (liquid). The base
film preferably has a refractive index substantially equivalent to that
of each of the first and second moth-eye layers and the transparent wall.
More specifically, the difference in refractive index among them is
preferably 0.05 or smaller (more preferably 0.03 or smaller), and in this
case, the reflection on the both interfaces can be hardly recognized.
From the same viewpoint, the difference in refractive index between
adjacent ones of these members is preferably 0.05 or smaller (more
preferably 0.03 or smaller).

[0100]The other optical characteristics of the base film are not
especially limited and may be the same as those of a base for common
optical films. The base film is preferably transparent to such a degree
that an observer can recognize the contents of the tank. More
specifically, it is preferable that the base film has a light
transmittance of 90% (more preferably 95%) or higher, a haze of 10%
(preferably 1%) or lower, and a refractive index of 1.55 to 1.45 (more
preferably 1.53 to 1.47).

[0101]The optical film of the present Embodiment is preferably a
transparent film with transparency, waterproof property, and resistances
to water and solvent (liquid). The optical characteristics of the entire
optical film of the present Embodiment are not especially limited. The
optical film is preferably transparent to such a degree that an observer
can recognize the contents of the tank. More specifically, it is
preferable that the optical film has a light transmittance of 90% (more
preferably 95%) or higher and a haze of 10% (more preferably 1%) or
lower.

[0102]Specific examples of the base film include ZEONOR by ZEON Corp.,
ARTON by JSR Corp., COC and TPX each by Mitsui Chemicals, Inc. Olefin
films are preferably used as the base film. The thickness of the base
film is generally, but not limited to, about 30 μm to 100 μm.

[0103]According to the embodiment (1), it is preferable that the optical
film of the present Embodiment has an adhesive paste on a (first or
second) moth-eye layer-free-side surface of the base film. The adhesive
paste (adhesive) is used for attaching the optical film of the present
Embodiment to the interior of the tank easily, and preferably has
transparency and water resistance. Specifically, the adhesive paste can
be disposed by coating an acrylic adhesive and the like on a single
surface of the base film by a publicly known method, and the thickness of
the paste can be appropriately determined. According to the embodiment
(1), the optical film of the present Embodiment may further include an
easy-peelable laminate film on the adhesive paste.

[0104]The embodiment (2) excludes use of the members such as the base film
and the adhesive paste, which permits a reduction in cost of the liquid
tank of the present Embodiment. Neither reflection nor reduction in light
transmittance caused by the base film and the adhesive paste occur,
resulting in further improvement in visibility of the liquid tank. The
embodiment (2) can be achieved by, for example, coating a UV-curable or
thermocurable resin directly on a transparent wall and then transferring
moth-eye structures of a soft mold to the resin coating by the above
nanoimprint.

[0105]It is preferable that the protective layer has a refractive index
substantially equivalent to that of a liquid (content liquid) to be
stored in the tank. According to this, the effects of the present
Embodiment can be more surely exhibited. More specifically, the
difference in refractive index between the protective layer and the
content liquid is 0.05 (more preferably 0.03) or smaller. In this case,
the reflection on the interface between the protective layer and the
content liquid can be hardly recognized. The refractive index of the
protective layer is preferably within a range of the refractive index of
the content liquid ±0.05 (more preferably ±0.03).

[0106]It is preferable that the protective layer has a refractive index
substantially equivalent to that of water. According to this, the liquid
tank of the present Embodiment can be preferably used as a tank for
storing water, i.e., as a water tank. More specifically, the difference
in refractive index between the protective layer and water is 0.05 (more
preferably 0.03) or smaller. In this case, the reflection on the
interface between the protective layer and the water can be hardly
recognized.

[0107]The refractive index of water is about 1.33, and so it is preferable
that the protective layer has a refractive index of 1.28 to 1.38 (more
preferably 1.3 to 1.36) in view of suitably using the liquid tank of the
present invention as a water tank.

[0108]According to the present Embodiment, the transparent members having
a refractive index of about 1.5 such as a glass or transparent resin
material can be preferably used as a material for the transparent wall.
Further, transparent members having a refractive index of about 1.5 such
as a transparent resin material can be suitably used as a material for
the first moth-eye layer. These transparent members each have a
refractive index higher than that of the content liquid (e.g., 1.33 of
water refractive index), generally. With a view to form the transparent
wall and the first moth-eye layer by the respective optimum materials and
to easily make the refractive index of the content liquid substantially
equivalent to that of the protective layer, it is preferable that the
protective layer has a refractive index smaller than that each of the
transparent wall and the moth-eye layer.

[0109]The other optical characteristics of the protective layer are not
especially limited. The optical film is preferably transparent to such a
degree that an observer can recognize the contents of the liquid tank.
More specifically, it is preferable that: the light transmittance is 90%
(more preferably 95%) or higher; and the haze is preferably 10% (more
preferably 1%) or lower.

[0110]It is preferable the protective layer contains a fluorine-containing
resin. According to this, the protective layer can more surely protect
the moth-eye structure, which is commonly poor in durability, and as a
result, the durability of the liquid tank of Embodiment 1 can be more
improved. Since the refractive index of the fluorine-containing resin is
generally about 1.3 to 1.4, the refractive index of the protective layer
containing a fluorine-containing resin can be easily made substantially
equal to that of water. That is, according to this, the liquid tank of
the present Embodiment can be preferably used as a water tank. The
protective layer can be formed by coating a material for the protective
layer on the first moth-eye layer by a common coating method (including,
spray, spin coating, die coating, slit coating, and the like), and drying
the coating if necessary.

[0111]The fluorine-containing resin is a resin including a carbon-fluorine
bond, and the resin itself has water repellency. CYTOP CTX 809A by ASAHI
Glass Co., Ltd., is mentioned as a specific example of the
fluorine-containing resin. The thickness of the protective layer is
generally, but not limited to, about 1 μm to 5 μm.

[0112]The adhesion of the protective layer possibly deteriorates when a
resin material (a polymer material) is used for the first moth-eye layer
and a fluorine-containing resin material (fluorine resin material) is
coated on the first moth-eye layer as a material for the protective
layer. In view of this, it is preferable that prior to the coating of the
protective layer material, the first moth-eye layer surface is treated
with a primer.

[0113]The flatness of the protective layer surface is not especially
limited as long as it is more flat than the moth-eye structure of the
first moth-eye layer. It is preferable that the protective layer surface
is flat to such a degree that stains on the surface can be removed by a
common washing way (e.g., wiping or rubbing) with a common tool (e.g.,
cloth or brush). There is no problem in that the protective layer surface
has a flatness that is allowed when the protective layer is composed of a
resin material by a common coating method such as spraying, spin coating,
die coating, and slit coating.

[0114]The transparent wall is transparent to such a degree that an
observer can view the under-liquid objects therethrough, and can be
composed of a material commonly used in typical water tanks or viewing
windows. More specifically, it is preferable that the transparent wall
has a light transmittance of 90% (more preferably 95%) or higher and a
haze of 10% (more preferably 1%) or lower. The refractive index of the
transparent wall is preferably, but not limited to, substantially
equivalent to that of the base film of the moth-eye layer or the resin
constituting the moth-eye structure. According to this, the effects of
the present Embodiment can be sufficiently exhibited.

[0115]Examples of the material for the transparent wall include, but not
limited to, various glass materials and transparent resin (e.g., acrylic
resin) materials. Such materials include those generally used for water
tanks or viewing windows.

[0116]The transparent wall constitutes the entire or part of the liquid
tank of the present Embodiment. Specifically, the transparent wall may be
a transparent window.

[0117]As long as the liquid tank is a container (appreciation or
observation container) the interior of which is viewed from the outside,
the application thereof is not especially limited. Particularly
preferable are aquarium tanks in which fishes are raised and bred and
tanks having as a liquid volume meter a window (viewing window) through
which the content is viewed.

[0118]The liquid (the content liquid) stored in the tank is in contact
with at least part of the protective layer, preferably. The kind and
refractive index of the content liquid can be appropriately determined
according to the application of the tank and are not especially limited.
Preferably, the content liquid is water. The content of the tank is not
especially limited to liquid materials and include underwater life such
as fishes and microorganism, and ornamentals.

Embodiment 2

[0119]The viewing device for under-liquid observation of Embodiment 2
includes a transparent window, and on the external surface of the window,
a first moth-eye layer having a moth-eye structure and a protective layer
covering the moth-eye structure are disposed in this order from the
window side.

[0120]The external surface of the transparent window means a surface on
the side opposite to the observer side.

[0121]Thus, in Embodiment 2, the first moth-eye layer and the protective
layer mentioned in Embodiment 1 are applied to a viewing device for
under-liquid observation. Embodiment 2 is mentioned below mainly with
reference to embodiments and effects different from those in Embodiment
1, and the same contents are omitted.

[0122]The viewing device of the present Embodiment is used with at least
the transparent window being in contact with liquid. Through this
transparent window, the observer views under-liquid objects.

[0123]The viewing device of the present Embodiment includes the first
moth-eye layer having a moth-eye structure, so that the reflection on the
interface between the first moth-eye layer and the protective layer can
be markedly decreased (e.g., a reflectance of 0.1% or lower). The
transparent window and the liquid in contact with the viewing device
(transparent window) (hereinafter, also referred to simply "liquid") are
usually different in refractive index. The material for the first
moth-eye layer can be appropriately selected from those having a
refractive index equivalent to that of the transparent window. The
material for the protective layer can be appropriately selected from
those having a refractive index substantially equivalent to that of the
liquid. Specifically, the first moth-eye layer ensures the refractive
index compatibility with the transparent window, and the protective layer
ensures that with the liquid. As a result, the refractive index can be
continuously changed from the liquid to the transparent wall, so that
reflection of ambient light on the external surface of the window can be
markedly decreased (e.g., a reflectance of 0.1% or lower).

[0124]For the same reasons as in Embodiment 1 the viewing device of
Embodiment 2 can exhibit the following effects:

[0125]coloring can be almost completely suppressed;

[0126]the durability of the external surface of the transparent window can
be improved; and

[0127]adhesion of stains to the moth-eye structure can be prevented.
Moreover, the shape of the liquid-contacting portion (surface) of the
protective layer is not especially limited, so that it can be made more
flat than the moth-eye structure. Therefore, even when adhered to the
protective layer, the stains can be readily removed therefrom.

[0128]Further, it is possible to produce the liquid tank dramatically
inexpensively in comparison to the case where other anti-reflection films
such as a multi-layer film (multi-layer anti-reflection film) are used.

[0129]It is preferable that the viewing device includes, on the internal
surface of the transparent window, a second moth-eye layer having a
moth-eye structure, and the second moth-eye layer is disposed in a region
facing the first moth-eye layer.

[0130]The internal surface of the transparent window means a surface on
the observer side.

[0131]This second moth-eye layer allows markedly reducing reflection of
ambient light on the window internal surface (e.g., a reflectance of 0.1%
or less). As a result of this, the reflection on the region where the
first and second moth-eye layers overlap with each other can be further
decreased. The first and second moth-eye layers are disposed so as to
overlap with each other in the viewing direction. Between the first and
second moth-eye layers, the shape, size, and location thereof may be the
same or different but preferably the same.

[0132]The location of the first and second moth-eye layers and the
protective layer is not especially limited as long as they can be viewed
from an observer. The first and second moth-eye layers and the protective
layer may be formed over the entire or part of the transparent window,
but preferably at least on a portion through which the user views the
observation objects. It is preferable that the first and second moth-eye
layers and the protective layer are disposed over the entire (surface) of
the transparent window in order to improve the visibility in a range as
wide as possible.

[0133]The materials for the first and second moth-eye layers are not
especially limited. It is preferable that the refractive index of each of
the first and second moth-eye layers is substantially equivalent to that
of the transparent window. More specifically, the difference in
refractive index between the transparent wall and the first and second
moth-eye layers is preferably 0.05 (more preferably 0.03) or smaller, and
in this case, the reflection on the both interfaces can be hardly
recognized.

[0134]The other optical characteristics of the first and second moth-eye
layers are not especially limited and may be the same as those of common
moth-eye films. The first and second moth-eye layers are preferably
transparent to such a degree that the user can view the observation
objects. More specifically, it is preferable that: the light
transmittance is 90% (more preferably 95%) or higher; the haze is
preferably 10% (more preferably 1%) or lower; the refractive index is
1.45 to 1.55 (more preferably 1.47 to 1.53).

[0135]Each of the first and second moth-eye layers may be (1) formed on
the transparent window with a base film therebetween or may be (2)
directly formed on the transparent window, from the same viewpoint as in
Embodiment 1.

[0136]Another Embodiment of the present invention provides such an optical
film. The present Embodiment provides an optical film attached to the
external surface of the viewing device of Embodiment 2, i.e., the viewing
device having a transparent window, the optical film including a moth-eye
layer having a moth-eye structure (the above-mentioned first moth-eye
layer) and a protective layer covering the moth-eye structure (the
moth-eye structure of the above-mentioned first moth-eye layer)

[0137]The base film is preferably transparent to such a degree that the
user can view the under-liquid objects. More specifically, it is
preferable that the base film has a light transmittance of 90% (more
preferably 95%) or higher, a haze of 10% (more preferably 1%) or lower,
and a refractive index of 1.45 to 1.55 (more preferably 1.47 to 1.53).

[0138]The optical film of the present Embodiment is preferably a
transparent film with transparency, waterproof property, and resistances
to water and solvent (liquid). The optical characteristics of the entire
optical film of the present Embodiment are not especially limited. The
optical film is preferably transparent to such a degree that the user can
view the under-liquid objects. More specifically, it is preferable that
the optical film has a light transmittance of 90% (more preferably 95%)
or higher and a haze of 10% (more preferably 1%) or lower.

[0139]According to the embodiment (1), it is preferable that the optical
film of the present Embodiment has an adhesive paste on a (first or
second) moth-eye layer-free-side surface of the base film, from the same
viewpoint as in Embodiment 1. The adhesive paste (adhesive) is used for
attaching the optical film of the present Embodiment to the viewing
device readily, and preferably has transparency and water resistance.

[0140]It is that the protective layer has a refractive index substantially
equivalent to that of a liquid to be in contact with the device.
According to this, the effects of the present Embodiment can be more
surely exhibited. More specifically, the difference in refractive index
between the protective layer and the liquid is 0.05 (more preferably
0.03) or smaller. In this case, the reflection on the interface between
the protective layer and the liquid can be hardly recognized. The
refractive index of the protective layer is preferably within a range of
the refractive index of liquid to be in contact with the viewing device
±0.05 (more preferably ±0.03).

[0141]The protective layer has a refractive index substantially equivalent
to that of water. According to this, the viewing device of the present
Embodiment can be preferably used as a device for observing underwater
objects, i.e., a device for underwater observation. More specifically,
the difference in refractive index between the protective layer and water
is 0.05 (more preferably 0.03) or smaller. In this case, the reflection
on the interface between the protective layer and the water can be hardly
recognized.

[0142]The refractive index of water is about 1.33, and so it is preferable
that the protective layer has a refractive index of 1.28 to 1.38 (more
preferably 1.3 to 1.36) in view of suitably using the viewing device of
the present Embodiment as a device for underwater observation.

[0143]According to the present Embodiment, the transparent members having
a refractive index of about 1.5 such as a glass or transparent resin
material can be preferably used as a material for the transparent window.
Further, transparent members having a refractive index of about 1.5 such
as a transparent resin material can be suitably used for the first
moth-eye layer. These transparent members each have a refractive index
larger than that of the liquid (e.g., 1.33 of water refractive index),
generally. With a view to form the transparent window and the first
moth-eye layer by the respective optimum materials and to easily make the
refractive index of the content liquid equivalent to that of the
protective layer, it is preferable that the protective layer has a
refractive index smaller than that of each of the transparent window and
the first moth-eye layer (transparent window).

[0144]The other optical characteristics of the protective layer is not
especially limited. The protective layer is preferably transparent to
such a degree that the user can view the under-liquid objects. More
specifically, it is preferable that: the light transmittance is 90% (more
preferably 95%) or higher; and the haze is preferably 10% (more
preferably 1%) or lower.

[0145]It is preferable the protective layer contains a fluorine-containing
resin. According to this, the protective layer can more surely protect
the moth-eye structure, which is commonly poor in durability, and as a
result, the durability of the viewing device of Embodiment 2 can be more
improved. Since the refractive index of the fluorine-containing resin is
generally about 1.3 to 1.4, the refractive index of the protective layer
containing a fluorine-containing resin can be easily made equal to that
of water. That is, according to this, the viewing device of the present
Embodiment can be preferably used as a device for underwater observation.

[0146]The transparent window is transparent to such a degree that the user
can view the under-liquid objects therethrough, and can be formed of a
lens or a transparent plate commonly used in typical goggles or
aquascopes. More specifically, it is preferable that the transparent
window has a light transmittance of 90% (more preferably 95%) or higher
and a haze of 10% (more preferably 1%) or lower. The refractive index of
the transparent window is preferably, but not limited to, substantially
equivalent to that of the base film of the moth-eye layer or the resin
constituting the moth-eye structure. According to this, the effects of
the present Embodiment can be sufficiently exhibited.

[0147]Examples of the material for the transparent window include, but not
limited to, various glass materials and transparent resin (e.g., acrylic
resin) materials. Such materials include those generally used for goggles
or aquascopes.

[0148]The transparent window constitutes the entire or part of the viewing
device of the present Embodiment.

[0149]As long as the user can view the under-liquid objects through the
transparent window of the viewing device, the application of the device
is not especially limited. Particularly preferable are a device for
observing under-liquid objects from above the liquid level and a goggle.
The former device preferably includes aquascopes. In the former device,
the transparent window may be a transparent plate, and in the latter one,
the transparent window may be a transparent lens. The viewing device may
be a window or wall for underwater observation with which ships, in-water
facilities, and the like are provided.

[0150]The kind and refractive index of the liquid are not especially
limited and may be appropriately determined according to applications of
the viewing device. Water, seawater, solvents, and the like are
mentioned.

Example 1

[0151]FIG. 1 is a schematic view showing a liquid tank of Example 1. FIG.
1(a) is an overall cross-sectional view of the tank. FIG. 1(b) is an
enlarged view of a region (side surface) surrounded by the broken line of
FIG. 1(a). FIG. 2 is a cross-sectional view schematically showing a
moth-eye layer of the liquid tank of Example 1. FIG. 3 is a
cross-sectional view schematically showing the moth-eye layer and a
protective layer in the liquid tank of Example 1.

[0152]The present Example relates to an embodiment in which the liquid
tank of the present invention is used as a water tank. The water tank of
the present Example has a multi-layer structure, as shown in FIG. 1,
composed of: a transparent wall 11 constituting a container storing a
content liquid, or water 21; a moth-eye layer (moth-eye film) 12 attached
to the external surface (hereinafter, also referred to as an "A surface")
of the transparent wall 11, which faces an observer 23; a moth-eye layer
(moth-eye film) 13 attached to the internal surface (hereinafter, also
referred to as a "B surface") of the wall 11; and a protective layer 14
coating the moth-eye layer 13. The transparent wall 11 is composed of a
transparent glass plate. The moth-eye layer 12 has a moth-eye structure.
The moth-eye layer 13 has a moth-eye structure and faces the moth-eye
layer 12. The protective layer 14 is composed of a fluorine-containing
resin (fluorine resin). The transparent wall 11 has a refractive index of
about 1.5. The protective layer 14 has a refractive index of 1.28 to
1.38. The water 21 has a refractive index of 1.33.

[0153]The moth-eye layer 12 can prevent a mismatch in refractive index
between air outside the water tank (refractive index of 1) and the
transparent wall 11. Also on the internal side of the wall 11, the
refractive index matching between the water 21 and the transparent wall
11 can be achieved. As a result, the observer 23 feels as if no wall of
the water tank exists and experiences a feeling of being inside the tank.
Further, when the water tank is photographed from the outside by strobe
photography, the strobe light is not reflected on the refractive index
interface. This can obviate the need of moving the camera close to the
tank. Thus, the matching in refractive index can be achieved well when
the moth-eye structure is formed on the B surface. On the other hand, the
moth-eye structure has a surface very excellent in biocompatibility, so
that underwater life easily adhere to the surface or stains on the
surface are difficult to remove. This problem can be solved by coating a
material whose refractive index is almost equivalent to that of the water
21 on the B surface of the moth-eye layer 13. As a result, the refractive
index matching between the water 21 and the transparent glass wall 11 is
achieved, and adhesion of stains including underwater life can be
minimized. Even if once adhered to the wall 11, the stains can be easily
removed therefrom with a commercially available cloth and the like.
Moreover, the fluorine-containing resin layer protects the surface of the
moth-eye layer 13 (the moth-eye structure), which is poor in durability,
whereby the water tank can be used as a durable aquarium or observation
tank. Since the refractive index of the fluorine-containing resin is
generally about 1.3 to 1.4, the refractive index of the protective layer
14 containing a fluorine-containing resin can be easily made
substantially equal to that of the water 21.

[0154]The moth-eye structure 16, with which the moth-eye layers 12 and 13
are provided, has protrusions. It is preferable that the protrusions are
formed with a pitch of 50 nm to 300 nm and a height of 50 nm to 300 nm.
The moth-eye layers 12 and 13 are each made of a material whose
refractive index is substantially equivalent to that of the material for
the transparent wall 11 of a glass or resin material.

[0155]In the present Example, CYTOP CTX809A (produced by ASAHI Glass Co.,
Ltd.) is used as the fluorine-containing resin material (fluorine coating
material) for the protective layer 14. This resin is coated on the
moth-eye layer 13 to form a layer with 0.5 μm in thickness. CYTOP
CTX809A has a refractive index of 1.34, which is almost equivalent to
that of water, or 1.33. Although in the single use of the moth-eye layer
13 the stains still easily adhere to the internal wall of the tank, the
fluorine resin coating on the internal wall of the tank suppresses the
stain adhesion and makes it easier to remove the stains from the wall.
The adhesion between the moth-eye layer 13 and the protective layer 14
might be insufficient when the moth-eye layer 13 is composed of a polymer
material and thereon the fluorine-resin coating is formed. In view of
this, it is preferable that the moth-eye layer 13 surface is treated with
a primer prior to the formation of the fluorine-resin coating. In this
Example, a primer (CT-P10, by ASAHI Glass Co., Ltd.) exclusively for
CYTOP CTX809A can be used.

[0156]Between the transparent wall 11 and each of the moth-eye layers 12
and 13, there are provided a base film 15 and an adhesive paste 17
disposed on the transparent wall 11-side main surface of the base film 15
as illustrated in FIG. 2. Thus each of the moth-eye layers 12 and 13 is
formed by subjecting to nanoimprinting a film of a UV-curable resin and
the like formed on the base film 15 and then attached to the transparent
wall 11 with the adhesive paste 17. The moth-eye structure 16 of the
layer 13 is coated with the protective layer 14 of a fluorine-containing
resin (fluorine resin) as illustrated in FIG. 3. As mentioned above,
attached is an optical film (film for the tank internal surface) composed
of the protective layer 14, the moth-eye layer 13, the base film 15, and
the adhesive paste 17 to the internal surface of the transparent wall 11;
and to the external surface thereof, an optical film (film for the tank
external surface) composed of the moth-eye layer 12, the base film 15,
and the adhesive paste 17 is attached. When a moth-eye layer is disposed
on the B surface side, the moth-eye layer 13, which is located on the
internal side of the tank, preferably includes a material with excellent
solvent resistance because it is under the presence of a solvent
(solution) such as water. The base film 15 preferably is a film with
transparency, water resistance, and solvent (liquid) resistance.
Specifically, olefin films, including the above-mentioned commercially
available ones, are suitably used. The refractive index of the base film
15 is preferably designed to be substantially equivalent to that of the
transparent wall 11.

[0157]FIG. 4 is an explanation view of refractive index change in the
liquid tank of Example 1. FIG. 4(a) is a side cross-sectional view of the
tank. FIG. 4(b) is a graph showing refractive index change on a side
surface of the tank. In the tank of the present Example, as shown in
FIGS. 4(a) and 4(b), the moth-eye layers 12 and 13 are disposed at a
portion facing the observer of the A and B surfaces of the transparent
wall 11 respectively whereby the refractive index matching can be ensured
with the solution (the water 21) being charged in the tank. Particularly
the refractive index matching on the B surface side is achieved using a
fluorine-containing material with a low refractive index for the
protective layer 14. Thus, the protective layer 14 can be coated
successfully while a mismatch of the refractive index on the interface
between the water and the resin layer is minimized. Further, the
treatment for the refractive index matching with the solution (the water
21) and the moth-eye structure enable the refractive index to
continuously change from air 22 (the refractive index is 1) to the water
21 (the refractive index is 1.33), and as a result, the portion facing
the observer of the wall 11 behaves as if no interface existed. Reflected
light on this portion have a very flat wavelength dispersion
characteristic and is less colored. Thus the coloring hardly occurs
because the moth-eye structure requires no λ/4 conditions unlike
the multi-layer film and because the refractive index continuously
changes from the surface of the moth-eye structure to the inside thereof.
The reflectance on the moth-eye structure surface can be extremely
decreased because the light does not recognize the moth-eye structure
surface as a refractive index interface. Moreover, the
fluorine-containing resin layer (with a refractive index substantially
equivalent to that of the solvent (e.g., 1.33 when the solvent is water))
is disposed on the B surface, which can ease the poor resistance to
stains such as underwater life and the difficulty in removal of the
stains of the nanostructure (nanometer-sized protrusions) of the moth-eye
structure and which can dramatically increase the durability of the B
surface.

Example 2

[0158]FIG. 5 is a schematic view showing the liquid tank of Example 2.
FIG. 5(a) is an overall cross-sectional view of the tank. FIG. 5(b) is an
enlarged view of a region (side surface) surrounded by the broken line of
FIG. 5(a).

[0159]The present Example employs exactly the principle of Example 1, but
the films for the tank internal surface and for the tank external surface
are each disposed on part of the tank by patterning. Specifically, as
shown in FIG. 5, the moth-eye layer 12 on the A surface, and the moth-eye
layer 13 and the protective layer 14 on the B surface, each of which is
disposed on the transparent wall 11 with the base film and the adhesive
paste therebetween, are patterned into the same planar shape. As a
result, the observer 23 feels as if no transparent wall 11 existed at the
pattern portion and he could enter the tank. Thus the display effects
like this can be obtained.

[0160]The pattern shape includes, but not limited to, a circular or square
shape in consideration of the viewing window shape. Alternatively, the
layers are patterned to express some character or logo for advertisement,
for example. When the layers are patterned to express some character, it
looks as if no interface exists on the layer-provided portion, but
therearound, light reflection occurs. This difference between the two
portions brings higher display effects.

Example 3

[0161]FIG. 6 is a perspective view schematically showing a liquid tank of
Example 3. FIG. 7 is an explanation view of refractive index change in
the liquid tank of Example 3. FIG. 7(a) is a cross-sectional view of a
liquid volume meter portion of the liquid tank. FIG. 7(b) is a graph
showing refractive index change in the liquid volume meter portion.

[0162]In the present Example, the liquid tank of the present invention is
applied to a tank equipped with a viewing window as a liquid volume meter
through which the liquid level can be determined. A liquid volume meter
18 is disposed at part of the tank, as shown in FIG. 6. The films for
tank internal and external surfaces are each attached to the liquid
volume meter portion, similarly to Example 1. More specifically, the
liquid volume meter portion of the tank of the present Example has a
multi-layer structure composed of: a transparent wall 11; a moth-eye
layer 12 attached to the external surface (A surface) of the transparent
wall 11 and having a moth-eye structure; a moth-eye layer 13 attached to
the internal surface (B surface) of the wall 11 and disposed in a region
facing the moth-eye layer 12; and the protective layer 14 coating the
moth-eye layer 13 and composed of a fluorine-containing resin (fluorine
resin) FIG. 7 illustrates that the level of the content liquid (the water
21) reaches a certain level of the liquid volume meter 18.

[0163]The B surface allows refractive index matching below the liquid
level (the water 21 level), but does not allow it thereabove, i.e., at
the portion exposing to air 22, as shown in FIG. 7(b). Specifically, the
reflection is suppressed in the region below the liquid level where the
refractive index is matched, and on the other hand, the reflection occurs
in the region thereabove where the refractive index is mismatched. This
difference in occurrence of the reflection makes it easy to determine the
height of the liquid level. The A surface is also provided with the
moth-eye structure, so that the external-surface reflection can be almost
completely prevented and as a result, the liquid level height can be more
easily determined.

[0164]When the protective layer 14 is not disposed on the moth-eye layer
13, the moth-eye structure is directly exposed to air above the liquid
level. This region also continuously changes the refractive index to
diminish the reflected light. As a result, the internal-surface
reflection is suppressed both above and below the liquid level, and the
interface between the liquid and air becomes unclear. In contrast to
this, when the protective layer 14 is disposed on the moth-eye layer 13,
the light reflection occurs on the interface between the air 22 and the
protective layer 14 above the liquid level. Thus, the liquid level can be
clearly determined as mentioned above.

[0165]When the liquid volume meter is not equipped with the films for tank
internal and external surfaces, the mismatch of the refractive index
occurs both above and below the liquid level. The degree of the mismatch
is different between the two regions, resulting in a difference in
reflectance therebetween. In this case, therefore, the liquid level is
not so clearly viewed as in the liquid volume meter of Example 3,
although barely determined.

[0166]As mentioned above, in Examples 1 to 3, the matching of refractive
index on the side surface of the transparent wall of the tank can be
achieved with the moth-eye structure and the resin with a low refractive
index. The light reflection can be suppressed as much as possible, and
thus objects (aquarium fishes, liquid level, and the like) inside the
tank can be clearly observed.

Example 4

[0167]FIG. 11 is a cross-sectional view schematically showing a viewing
device for under-liquid observation of Example 4. FIG. 12 is an
explanation view of refractive index change in the viewing device of
Example 4. FIG. 12(a) is a cross-sectional view of the device. FIG. 12(b)
is a graph showing refractive index change in the device. FIGS. 13 and 14
are cross-sectional views each schematically the device of Example 4 in
accordance with a modified example.

[0168]In the present Example, the viewing device of the present invention
is applied to a device through which a user views under-liquid objects
above the liquid level. The device of the present Example has the
following multi-layer structure composed of: a transparent plate-like
base (transparent window) 24; a moth-eye layer 12 attached to the surface
exposed to air (the top) of the base 24 and having a moth-eye structure;
a moth-eye layer 13 attached to the liquid (water)-side surface (the
bottom) of the base 24 and disposed in a region facing the moth-eye layer
12; and a protective layer 14 of a fluorine-containing resin (fluorine
resin) coating the moth-eye layer 13. The base 24 is made of a glass or
resin material and the like, and has a refractive index of about 1.5. The
device of the present Example is used with the protective layer 14 being
in contact with the liquid and the moth-eye layer 12 being in contact
with the atmosphere. Accordingly, it is preferable that the respective
members of the present Example are made of materials with a specific
gravity lower than that of the liquid.

[0169]In the present Example, the moth-eye films are attached to the
respective surfaces of the base 24, and on the moth-eye film that is to
be exposed to liquid, the protective layer 14 of a resin with a low
refractive index is disposed. When this device is floated on the surface
of the liquid, as shown in FIG. 12, the both surface allow the refractive
index matching, and the refractive index continuously changes from air
(refractive index of 1) to water 21 (refractive index of 1.33). Thus the
refractive index mismatch on the liquid surface can be solved. As a
result, underwater objects can be clearly viewed from above the liquid
level. Further, since this Example employs the fluorine resin as a
material for the protective layer 14, similarly to Examples 1 to 3, the
device can suppress from being contaminated by the liquid and can achieve
the refractive index matching also on the bottom thereof.

[0170]A float 25 with a lower specific gravity than that of the liquid may
be disposed around the device of the present Example (for example, around
the base 24), as shown in FIG. 13. This allows using materials with a
higher specific gravity than that of the liquid for the members such as
the base 24.

[0171]The device of the present Example may be equipped with a
light-shielding cylindrical member 26 as shown in FIG. 14 to be used as
an aquascope. One opening of the cylindrical member 26 is closed with a
transparent portion such as the base 24. This can suppress incident of
ambient light into the transparent portion so that observation objects
can be more clearly viewed. Further, the gravity of the cylindrical
member 26 can be made lower than that of the liquid, whereby materials
with a specific gravity higher than that of the liquid can be used for
the members such as the base 24.

[0172]The application of the device of the present Example is not
especially limited. The device may be used for underwater or undersea
observation, or alternatively, for viewing an indicator inside an
apparatus containing a solvent or checking behavior of such an apparatus.

Example 5

[0173]FIG. 15 is a cross-sectional view schematically showing a viewing
device for under-liquid observation of Example 5. FIG. 16 is an
explanation view of refractive index change in the device of Example 5.
FIG. 16(a) is a cross-sectional view of the device. FIG. 16(b) is a graph
showing refractive index change in the device.

[0174]In the present Example, the viewing device of the present invention
is applied to goggles. The lens part of the goggles of the present
Example has the following multi-layer structure composed of a transparent
base (transparent window) 27; a moth-eye layer 12 attached to the
internal surface of the base 27 (the user 27-side surface) and having a
moth-eye structure; a moth-eye layer 13 attached to the liquid (water
21)-side surface (the external surface) thereof and disposed in a region
facing the moth-eye layer 12; and the protective layer 14 of a
fluorine-containing resin (fluorine resin) coating the moth-eye layer 13.
The base 27 is composed of a glass or resin material, and the like, and
has a refractive index of about 1.5. The goggles of the present Example
are used with the protective layer 14 being in contact with the liquid
and the moth-eye layer 12 being with air.

[0175]In the present Example, the moth-eye films are attached to the
respective surfaces of the base 27, and on the moth-eye film that is to
be exposed to liquid, the protective layer 14 of a resin with a low
refractive index is disposed. In underwater observation by means of the
goggles of the present Example, as shown in FIG. 16, the both surface can
achieve the matching of the refractive index, and the refractive index
can be continuously changed from the water 21 (the refractive index is
1.33) to air (the refractive index of 1), finally to just in front of
user's (observer's) eyes. The user can view objects present in the water
naturally and clearly like fishes do.

[0176]The moth-eye layer 12 surface may be provided with a water-shedding
treatment to such a degree that the moth-eye structure does not
completely lose its undulated pattern, although the above effects can be
exhibited without the treatment. This allows easy removal of water from
the lens inner surface.

[0177]The moth-eye layer 12 surface may be provided with hydrophilicity.
In this case, even when water enters the inside of the lens part and
comes in contact with the moth-eye layer 12, the water can be spread over
the layer 12 surface to form a water film thereon. Thus, fogging of the
goggles can be suppressed.

[0178]The goggles of the present Example may be those for athletic use,
hobby use, or fishing industries, and the application thereof is not
especially limited.

[0179]Also in Examples 4 and 5, a base film and an adhesive paste are
disposed in this order on the base 24 or 27-side main surface of each of
the moth-eye layers 12 and 13, similarly to Examples 1 to 3. Thus, the
optical film composed of the protective layer 14, the moth-eye layer 13,
the base film, and the adhesive paste is attached to the bottom of the
base 24 or the external surface of the base 27, and on the top of the
base 24 or the internal surface of the base 27, the optical film composed
of the moth-eye layer 12, the base film, and the adhesive paste is
attached.

[0180]The present application claims priority to Patent Application No.
2008-329996 filed in Japan on Dec. 25, 2008 under the Paris convention
and provisions of national law in a designated State, the entire contents
of which are hereby incorporated by reference.